When Prisoner 106 arrived at the maximum security facility of Level Six in the New Mexico State Penitentiary in Santa Fe, he was on the books as a dangerous armed robber headed straight for solitary confinement.

But as he was moved into his 12-foot-by-7-foot cell, where he would have little to no physical contact with the outside world, the thing that almost no one at the prison knew was that this inmate was actually the Secretary of Corrections for the state of New Mexico.

Gregg Marcantel is a former cop who spent a career putting away criminals, and he decided to put himself in solitary in Level Six to live among the worst of the worst inmates.

“This is where our predators, our monsters, are dangerous inmates are housed,” said Alex Tomlin, the public affairs director for New Mexico's Department of Corrections. “My greatest fear is that something happens to [Marcantel].”

ABC News

Gregg Marcantel, the Secretary of Corrections for the state of New Mexico, spent 48 hours undercover in solitary confinement at the New Mexico State Penitentiary, outside of Santa Fe, New Mexico.

“[Even] 24 hours in a maximum security prison is still a lot of time,” she continued. “The next 48 hours will really just be about waiting and praying.”

There are an estimated 80,000 inmates locked up in solitary confinement in the United States, but this form of incarceration has become extremely controversial. Not only is it three times as expensive to house an inmate in solitary, but critics say it can also make inmates more dangerous.

Marcantel said he was doing this because he wants to reform the state's use of solitary. He said most of the inmates who are placed in solitary confinement will end up back on the streets, transformed into super criminals.

“We are sending them back to our neighborhoods worse than when they came... I can’t allow that,” he said.

But before he made any changes, Marcantel wanted to experience solitary from the inside, so he spent 48 hours living in lockdown in Level Six.

Level Six is divided up into three separate housing units, holding a total of 282 inmates, who are doing time for crimes that range from rape to armed robbery to murder. Combined, the men here have killed 138 people.

Once in his cell, Marcantel sorted through his prison-issued personal effects and opened a letter from his wife.

“The whole time we’ve ever been together, there’s never been a day that we didn’t speak to one another,” he said.

Very quickly, the inmates started to buzz about the latest arrival, yelling “hey, new guy” and “Cell 6, who the [expletive] are you?” Correction officers told the inmates that Prisoner 106 was speech and hearing impaired because the secretary worried that if the inmates hear his distinctive voice, his cover will be blown and his security compromised.

As the hours rolled by, the secretary started to realize that the greatest danger wasn’t from other inmates, but from his own mind.

Several inmates told “Nightline” the only way to mentally survive in solitary confinement, which they sometimes referred to as “segregation” or simply “seg,” is to have a routine.

“I exercise. I read a lot. If I didn’t have books I probably would have already gone insane,” said Freddy Munoz, who was one housing unit over from Marcantel.

Growing up, Munoz, 34, said he wanted to be an astronaut, but at age 13 he got caught up in gangs and committed two murders. Prison officials have kept him in a cell in solitary confinement every day for years because of his violent past and gang affiliations. He knows every crack and every inch of peeling paint in his cell.

On the other side of Level Six was Daniel Herrera, who was also placed in solitary because he has been a gang member. He is 23 years old and serving a sentence for kidnapping.

Herrera showed how he and other inmates “go fishing” by pulling and throwing a wire to get things from one cell to the next, “without getting caught,” he said, although the correction officers said they were well aware of the fishing phenomenon. If they are caught, Herrera said inmates can lose privileges like rec time.

“You’ve got 23 hours a day locked down, you got a lot of time to come up with,” Herrera said.

After six hours in solitary, Marcantel was served his first meal: a meat patty, baked beans, wheat bread and cauliflower. After nine hours, he brushed his teeth and got ready for a long night. The next morning, he was served plain pancakes.

“I’ve developed what I call an ‘inmate clock,’” Marcantel said. “I would touch the window and if it were cool and light, I knew it was morning… if it was warm, I knew it was afternoon.”

When collecting food trays, the corrections officers did a check to make sure no one broke off a piece of plastic that could be used to create a weapon.

“It takes a certain kind of person [to do this job],” said one officer. “We’re working inside the lion’s cage with the lion.”

To keep themselves safe, the officers follow rigid protocols. For example, they strip search inmates for weapons or contraband any time they take one out to the yard, which is known as the “rec pen.”

“I’m feeling a little nauseated and I’m standing at the door now and I’m looking outside and I’m realizing that I can’t get on the other side of the door,” he said. “I felt like the cell was kind of squeezing down on me."

Critics of solitary confinement call it legalized torture. They say studies show it can result in brain damage similar to that made by head trauma. Humans are social animals and we are built for interaction. Take that away, and critics say inmates can literally lose their minds.

But many corrections officials say solitary confinement is a necessary tool to control a dangerous prison population. Officers at the New Mexico prison point to inmate Nathaniel Stein as an example. Stein was placed in solitary after he viciously attacked a corrections officer a few months ago.

“I don’t really feel bad for the things I’ve done,” Stein said.

On the flipside, there are inmates in solitary like Freddy Munoz, who has been a model inmate, despite the severity of his crimes. Munoz was one of a select few handpicked inmates to get out of solitary and into the general population of the prison as part of Secretary Marcantel’s plan -- the restoration into population program -- to reduce the number of inmates in solitary.

“To be able to walk around a little bit more... to be able to go outside without having to be handcuffed and restrained every day, I think it would be very good for me,” Munoz said.

When Marcantel was released, the first thing he reached for was a large cup of coffee.

“Not sure I want to do it again, but I am really glad I did it,” he said.

After spending 48 hours on the inside, the secretary said he believes that soliatry should continue to be used but only sparingly, for the most dangerous inmates.

“There is such a thing as evil,” he said. “It’s not whether or not they are going to hurt people—they started hurting people when they were four years old -- and it’s up to me to make sure that not only am I protecting other inmates from them, but I am also protecting my staff from them.”

Sure enough, a few days after Marcantel release, Munoz, Daniel Herrera and six other inmates were brought out of Level Six solitary and into an intermediate level, in preparation for an eventual return to the general prison population.

“This is living, right here, finally,” Munoz said.

For the first time in years, they will actually be able to sit next to one another and have real human contact. These men are part of an experiment and part of a trend as New Mexico and the country takes its first steps away from the costly and controversial form of punishment.